Subsea structure and methods of construction and installation thereof

ABSTRACT

A structure ( 200 ) for subsea installation has buoyancy distributed substantially along its length in order to float said structure in horizontal orientation to a point of installation. When said structure is upended, said distributed buoyancy maintains said structure in a substantially vertical orientation and operational state, without the need for further, non-distributed buoyancy (for example, without substantial tension being provided by a top buoy or surface vessel ( 100 )). The structure may comprise a flexible or rigid riser conduit for hydrocarbons. Buoyancy modules may be fixed with spaces between them, for example to achieve a structure similar to a SLOR, or COR, but without a top buoy. The buoyancy modules may alternatively be free to slide longitudinally along the structure, so as to impart their buoyancy force through one another to the top of the structure.

The present invention relates to method and apparatus for buoyancy tensioning of offshore deepwater structures. It finds particular application in tensioning a slender, vertical or near-vertical, bottom-anchored, submarine structure, such as a riser or a bundle of risers (which may, or may not, include a structural member) or an umbilical.

Tensioning is the act of ensuring that a marine structure doesn't experience excursions from its nominal upright position that would fall outside the acceptable limits, even in extreme weather conditions, the said limits being possibly defined with reference to the occurring seastate. There should always be sufficient tension to ensure stability, no matter the weight of the structure and the weight of the pipelines/risers hanging off the structure.

The structure may form part of a so-called hybrid riser, having an upper and/or lower portions (“jumpers”) made of flexible conduit. U.S. Pat. No. 6,082,391 (Stolt/Doris) proposes a particular Hybrid Riser Tower consisting of an empty central core, supporting a bundle of riser pipes, some used for oil production some used for water and gas injection. This type of tower has been developed and deployed for example in the Girassol field off Angola. Insulating material in the form of syntactic foam blocks surrounds the core and the pipes and separates the hot and cold fluid conduits. Further background has been published in paper “Hybrid Riser Tower: from Functional Specification to Cost per Unit Length” by J-F Saint-Marcoux and M Rochereau, DOT XIII Rio de Janeiro, 18 Oct. 2001. Updated versions of such risers have been proposed in WO 02/053869 A1.

GB-A-2346188 (2H-Offshore) presents an alternative to the hybrid riser tower bundle, in particular a “concentric offset riser” (COR). The riser in this case includes a single production flowline located within an outer pipe. Other lines such as gas lift, chemical injection, test, and hydraulic control lines are located in the annulus between the core and outer pipe. The main flow path of the system is provided by the central pipe and the annular space may be filled with water or thermal insulation material. Water injection lines, which are generally equal in diameter to the flowline, are not accommodated and presumably require their own riser structure. A simpler single line offset riser (SLOR) is also marketed by 2H Offshore Engineering Limited.

Prior solutions are based on the use of a top buoy that gives part (or all) of the required tension to the slender structure, which, at its other extremity, is anchored to seabed. This top buoy can be provided with a restoring moment in order to thwart any kind of moments induced by other attached hardware, such as connecting jumpers, mooring cables, etc. . . . Furthermore, this top buoy can be tied to the other parts of the structure through a rigid connection (transmitting moments) or through a hinge (3D or at least 2D). The slender structure can also be endowed with distributed buoyancy, to provide support when horizontal (so that it can be towed, for example).

Provision of buoyancy tanks and floating operations to connect to the main support structure are expensive and a major constraint on project planning. With the current designs, connection of the top buoy to the other parts of the structure, be it conducted onshore or offshore, is a difficult operation, and time and resource-consuming.

Furthermore, if the buoy is rigidly linked to the other parts of the slender structure, the connecting part is likely to be subject to high stresses and/or fatigue, depending on the way the complete structure is installed, the environmental conditions on site, etc. . . .

As a consequence, it is an object of the invention to provide a method of providing buoyancy to a floating structure that addresses one or more of the aforementioned disadvantages.

In a first aspect of the invention there is provided a structure for subsea installation having buoyancy distributed substantially along its length in order to float said structure in horizontal orientation to a point of installation, whereby when said structure is upended, said distributed buoyancy will maintain said structure in a substantially vertical orientation and operational state, without the need for further, non-distributed buoyancy (for example, without substantial tension being provided by a top buoy or surface vessel).

In a second aspect of the invention there is provided a method of installing an elongate subsea structure having buoyancy distributed substantially along its length in order to float said subsea structure to the point of installation, wherein when said structure is upended and anchored to the seabed, either directly or indirectly, said distributed buoyancy is sufficient to maintain said structure in a substantially vertical orientation without a substantial further separate buoyant structure.

The invention yet further provides such a structure in its installed state. This technique removes the need for a top buoy, replacing it with distributed buoyancy on the risers and structure. Distributed buoyancy is of course known for the shaping of portions of risers and like structures, for example to control touchdown behaviour in so-called “wave” configurations. This is generally with the context of a catenary structure suspended from a substantial vessel.

Said distributed buoyancy may be provided by buoyancy modules provided at regular intervals along said rigid or flexible risers. This may involve fixing the modules with spaces between them, for example to achieve a structure similar to a SLOR, or COR, but without a top buoy.

The buoyancy modules may alternatively be free to slide longitudinally along the structure, so as to impart their buoyancy force through one another to the top of the structure, rather than being transmitted to the supported structure at points along the length thereof. This alternative is preferred in accordance with the invention of a further patent application having the same priority date as the present application. The content of that other application is incorporated herein by reference (GB 0227851.3, agent's ref 64312GB, published as WO ______).

Said subsea structure may comprise at least one rigid riser conduit. Said structure in its operational state may be connected to a flexible jumper or similar flexible portion at its top end for connection to a source or destination of fluid. Only a lesser proportion of said flexible jumper may have distributed buoyancy. Said buoyancy may be distributed in such a way to give the flexible portion a “steep wave” shape, in order to apply tension co-linear with the structure, thus avoiding large bending moments in the subsea structure.

Said subsea structure will usually be a riser, or a bundle of risers. Said bundle of risers may comprise a plurality of spaced risers surrounding a central riser or support.

Said buoyancy modules may be spaced substantially regularly along said subsea structure to enable said structure to be floated to the deployment site in a substantially horizontal orientation prior to deployment.

The distribution of buoyancy about the length of the riser may be selected to provide substantially normal orientation of said flexible portions as they attach to said structure, such that any bending moment induced by tension in said flexible risers is minimised.

The distributed buoyancy referred to herein will generally be of a permanent type, such as syntactic foam. Adjustments to the buoyancy may be made temporarily by the addition of modules, or in particular by air-filling then flooding parts of the structure at different stages in the installation process.

Dense material may be used to fill the structure during deployment in order to compensate for the extra buoyancy required by the invention. Said dense material may comprise seawater, or may be a material denser than the ambient seawater. In the case of an onshore-fabricated, towed and upended structure, the downward tension that must be applied during upending at the bottom of the structure must overcome the net buoyancy of the structure. In such a case, once in operation, the tension of the structure (as measured for example, at anchor) is given by this net buoyancy plus the difference in content weight between the upending and operating phases. For example, if a gas riser is installed full of water, when in operation, it will be further tensioned by the difference in density between gas and water. The use of even heavier fluids, such as drilling mud, may be envisaged during installation, in order to boost the net buoyancy provided once in operational conditions by adding a bigger quantity of buoyancy material than can be properly compensated during installation.

The finished installation may further comprise flexible jumpers with distributed buoyancy, whose buoyancy contributes to the tension in the structure in its operational condition. By deferring the connection of these jumpers until after upending and anchoring the main part of the structure, the net buoyancy during upending can be further reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which:

FIG. 1 shows a known type of riser structure in an offshore oil production system;

FIG. 2 shows two riser bundles each having a novel structure without top buoy, in accordance with an embodiment of the present invention; and

FIG. 3 shows in more detail the structure of the bundles in the embodiment of FIG. 2, including sliding buoyancy modules.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 illustrates a floating offshore structure 100 fed by riser bundles 110, which are supported by subsea buoys 115. Spurs 120 extend from the bottom of the riser bundle to the various well heads 130. The floating structure is kept in place by mooring lines (not shown), attached to anchors (not shown) on the seabed. The example shown is of a type known generally from the Girassol development, mentioned in the introduction above.

Each riser bundle is supported by the upward force provided by its associated buoy 115. Flexible jumpers 135 are then used between the buoys and the floating structure 100. The tension in the riser bundles is a result of the net effect of the buoyancy combined with the ultimate weight of the structure and risers in the seawater. The skilled person will appreciate that the bundle may be a few metres in diameter, but is a very slender structure in view of its length (height) of for example 500 m, or even 1 km or more. The structure must be protected from excessive bending and the tension in the bundle is of assistance in this regard.

FIG. 2 shows two risers 200, 210 of novel structure connecting to a floating structure 100. Any additional top buoyancy has been replaced with distributed buoyancy. The figure shows the structure subject to a positioning excursion (caused by the sea-state, for example). The left-hand riser is under greater tension than the right hand riser, but use of flexible top sections 220 allow the risers to accommodate the transitions. As mentioned in the introduction, each riser may be a single conduit, but in the present example is assumed to be a substantial bundle of conduits, and the flexible top section a corresponding bundle (umbilical) of flexible conduits.

Buoyancy for the risers is provided by the buoyancy already distributed along them for installation purposes, evenly distributing the complement required in operational conditions. It may be desirable to compensate for any surplus of buoyancy, during installation, by filling the structure with fluids heavier than those in operation.

A consequence of the absence of a top buoy is that the structure supported in such a manner cannot withstand a large bending moment at top, since the only counteracting stiffness is given by the steel and is therefore very low, due to the slenderness of the structure. Using flexible sections 220 to connect to the top of the structure overcomes this problem, and providing them with a steep-wave shape, in order to apply tension co-linear with the structure, avoids a large bending, or rotational, moment. Each flexible section 220, at least in the region 230 above the junction 240 with the rigid portion 200/210, is also provided with distributed buoyancy for this purpose. The tension given by the flexibles, during operation, is taken into account when determining the buoyancy required along the structure itself (for example with regard to safety margins, installation time and other parameters). However, in the preferred embodiment, the buoyancy and tension in the flexible sections is not required to support the main part of the structure, and installation (upending) is conducted prior to attachment of the jumpers to the top of the structure.

Note that a catenary configuration for a particular jumper can be considered if the induced bending moment is acceptable. For example, the gas-lift jumpers (9 cm, approximately 3.5″), which are light, could be in a catenary configuration Their installation will be straightforward (similar to Girassol jumpers).

FIG. 3 is a side view of one of the risers 200 in schematic detail. The riser comprises a bundle of riser conduits 300 around a structural core 310 to provide support. The embodiment illustrated shows the buoyancy modules 320 free to slide along the risers, as described in our co-pending patent application (Agent's reference 64312 GB). Inter-module devices 330 between buoyancy modules are used to provide a suitable interface between the surfaces of adjacent modules. All of the buoyancy thrust is applied to the top of the structure, which may, in the case of the embodiment described with reference to FIG. 2, be a junction 240 between rigid 200 and flexible 220 riser bundles.

In alternative embodiments, the modules 320 may be spaced along the riser 200, and anchored to the core 310. While this is particular suitable for a single conduit (SLOR) riser, however, it becomes less desirable when a number of heavy riser conduits 300 are hanging from the top plate, and imparting a compression force to the core 310.

A fabrication/installation sequence for this type of riser may comprise the following steps:

-   -   1. Fabrication onshore, empty.     -   2. Launching (progressively during fabrication or in one         operation), preferably in a still water area.     -   3. Flooding with the selected fluids (performed progressively         during launching and fabrication or in one operation).     -   4. Surface or subsurface towing to site.     -   5. Upending: the total net buoyancy of the structure cannot         exceed the winch capacity and/or the bollard pull of the         installation vessel.     -   6. Anchoring (transfer of tension).     -   7. Fluid replacement (if any), which further tensions the         structure.     -   8. Jumper connection, which further tensions the structure.

The skilled person will further appreciate that the exact form of components and methods used can vary from the ones described herein without departing from the spirit and scope of invention. 

1. A structure for subsea installation, the structure comprising at least one rigid riser conduit extending substantially the full length of the structure for transfer of fluid between the seabed and sea surface, the elongate structure having buoyancy distributed substantially along its length, the quantity and distribution of said buoyancy being sufficient both to float said structure in horizontal orientation to a point of installation, and when said structure is upended, to maintain said structure in a substantially vertical orientation and operational state.
 2. A structure as claimed in claim 1, wherein said distributed buoyancy is provided by buoyancy modules provided at regular intervals along said subsea structure.
 3. A structure as claimed in claim 2, wherein said buoyancy modules are fixed with spaces between them.
 4. A structure as claimed in claim 3, wherein said buoyancy modules are spaced substantially regularly along said subsea structure to enable said structure to be floated to the deployment site in a substantially horizontal orientation prior to deployment.
 5. A structure as claimed in claim 1, wherein said buoyancy modules are free to slide longitudinally along the structure, so as to impart their buoyancy force through one another to the top of the structure, rather than being transmitted to the structure at points along the length thereof.
 6. A structure as claimed in any one of claims 1 or 2, wherein said structure in its installed state is connected to at least one flexible jumper or similar flexible portion at its top end to connect with said riser conduit a source or destination of fluid.
 7. A structure as claimed in claims 6, wherein said flexible jumper is provided with distributed buoyancy, whose buoyancy contributes to the tension in the structure in its installed state.
 8. A structure as claimed in claim 7, wherein only a lesser proportion of said flexible jumper has distributed buoyancy.
 9. A structure as claimed in claim 6, wherein said buoyancy is distributed in such a way to give said flexible portion a “steep wave” shape, in order to apply tension co-linear with the structure, thereby avoiding large bending moments in the structure.
 10. A structure as claimed in claim 6, wherein the distribution of buoyancy about the length of the structure is selected to provide substantially normal orientation of said flexible portions as they attach to said structure, such that any bending moment induced by tension in said flexible portions is minimise.
 11. (canceled)
 12. A structure as claimed in any one of claims 1 or 2 comprising a plurality of spaced risers surrounding a central core.
 13. A structure as claimed in claim 12, wherein said also serves as a riser conduit.
 14. A structure as claimed in claim 12, wherein said core comprises a support for said riser conduit.
 15. A structure as claimed in any one of claims 1 or 2, wherein said distributed buoyancy is of a permanent type, such as syntactic foam.
 16. A method of installing an elongate subsea structure comprising a riser conduit for transfer of fluid between the seabed and sea surface and having buoyancy distributed substantially along its length in order to float said subsea structure to the point of installation, wherein when said structure is upended and anchored, either directly or indirectly, to the seabed, said distributed buoyancy is sufficient to maintain said structure in a substantially vertical orientation without a substantial further separate buoyant structure.
 17. A method of installing an elongate subsea structure as claimed in claim 16, wherein said riser conduit in its installed state is connected to at least one flexible jumper or similar flexible portion at its top end for connection to a source or destination of fluid.
 18. A method of installing an elongate subsea structure as claimed in claim 17, wherein said flexible jumper is provided with distributed buoyancy, whose buoyancy contributes to the tension in the structure in its installed state.
 19. A method of installing an elongate subsea structure as claimed in claim 18, wherein only a lesser proportion of said flexible jumper is provided with distributed buoyancy.
 20. A method of installing an elongate subsea structure as claimed in any one of claims 17 to 19, wherein said buoyancy is distributed in such a way as to give said flexible portion a steep wave shape, in order to apply tension co-linear with the structure, thereby avoiding large bending moments in the subsea structure.
 21. A method of installing an elongate subsea structure as claimed in any of claims 17 to 19, wherein the distribution of buoyancy about the length of the riser is selected to provide substantially normal orientation of said flexible portions as they attach to said structure, such that any bending moment induced by tension in said flexible risers is minimised.
 22. A method of installing an elongate subsea structure as claimed in any one of claims 16 to 19, wherein adjustments to said distributed buoyancy are made temporarily by the addition of subsequent modules.
 23. A method of installing an elongate subsea structure as claimed in any one of claims 16 to 19, wherein adjustments to said distributed buoyancy are made by air-filling then flooding parts of the structure at different stages in the installation process.
 24. A method of installing an elongate subsea structure as claimed in any one of claims 16 to 19, wherein dense material is used to fill the structure during installation in order to compensate for the extra buoyancy required to perform said method of installation.
 25. A method of installing an elongate subsea structure as claimed in claim 24, wherein said dense material comprises seawater.
 26. A method of installing an elongate subsea structure as claimed in claim 24, wherein said dense material comprises a material denser than the ambient seawater.
 27. An installed structure as claimed in any of claims 16 to 19, whereupon said structure has been upended and said distributed buoyancy maintains said structure in a substantially vertical orientation and operational state, without the need for further, non-distributed buoyancy. 